1. Field of the Invention
This invention concerns delignification and bleaching of cellulosic material with peroxides in an alkaline medium.
2. Prior Art
Cellulose pulped by acid sulfite or alkaline soda or sulfate (Kraft) processes contains residual lignin, hemicellulose and several other materials. These materials are associated with the cellulose and are primarily responsible for discoloration or yellowing of cellulose or products produced therefrom. In order to produce very white, bright pulp, kraft and sulfite pulps are bleached by a multi-step bleaching process.
Whitening and delignifying pulp by a multi-step bleaching process can also have deleterious effects upon the pulp depending upon the harshness of the bleaching processes. The beneficial and deleterious effects upon pulp are determined by various standard tests. The amount of delignification is indicated by a decrease in the permanganate number. Brightness is indicated by brightness number tests. Change in strength is indicated by the test for pulp viscosity.
The tests reported herein are:
Potassium permanganate number (K-number) as determined by TAPPI standard method T 214 M42.
Brightness as measured on a General Electric photometer in accordance with TAPPI standard T-217m and expressed in terms of percent brightness units. Reverted or aged brightness is determined by re-reading the brightness after the sheets have been heated at 105.degree. C. for one hour in an air circulating oven.
Viscosity of the pulp as determined in accordance with TAPPI standard T-230 and reported in terms of centipoise.
Hand sheets are made for testing in accordance with the procedure described in TAPPI standard T-218m for optical tests.
Reduction in the K-number indicates delignification and is considered beneficial. An increase in the brightness number indicates improved whiteness of the pulp and is considered beneficial. Higher numerical values for the viscosity tests indicate less degradation of the pulp during bleaching and delignification and therefore a better bleaching sequence.
Individual steps in a multi-step bleaching process for removing residual lignins and whitening the pulp are well known and generally employ chemicals such as chlorine, chlorine dioxide, sodium or calcium hypochlorite, alkaline extractions, oxygen, ozone and peroxides.
Multi-step bleaching processes employing conventional bleaching chemicals comprise a series of steps, which usually employs chlorine. There has been considerable interest recently in solving the serious problems in chemicals recovery and in the disposal of waste materials associated with chlorine-containing bleaching agents. These difficulties can obviously be avoided by using bleaching agents which do not contain chlorine such as peroxides. These are advantageous from the standpoint of eliminating the pollution and corrosion problems associated with chlorine bleaching, however, heretofore the use of peroxides has not been widely adopted for this purpose because of its expense and ineffectiveness in delignification. Consequently it has typically been used near the end of a bleaching sequence after most of the lignin has already been dissolved out of the pulp by other bleaching agents.
Multi-step bleaching with highly alkaline peroxygen bleaching steps is described in prior art patents, for example, U.S. Pat. No. 3,865,685 (Hebbel et al.) granted Feb. 11, 1975 and U.S. Pat. No. 2,779,656 (Fennell) granted Jan. 29, 1957. Fennell at column 4, lines 67-70 teaches that a peroxygen compound in the liquor for the caustic extraction has a two-fold effect; it bleaches and at the same time increases the effectiveness of the caustic extraction.
It is further recognized by the prior art that the peroxide in an alkaline bleach liquor can be catalytically decomposed by heavy metals such as copper, iron and manganese which are frequently found in the water used by the pulp mill. See for example U.S. Pat. No. 2,920,011 granted Jan. 5, 1960, to Eilers at column 3, lines 32-36.
To protect or stabilize the peroxide it has been customary to add inorganic complexing agents or stabilizers such as sodium silicate ("water glass") or magnesium sulfate or organic complexing agents such as ethylene-diamine tetracetic acid ("EDTA"). See for example Hebble et al at column 3, lines 13-36.